RFR: 8371183: RichTextModel: ContentChange.isEdit incorrect undoing style modification

[Andy Goryachev]

Fixes an issue found by an external user:

Undoing a style only change fires a change event where `ContentChange.isEdit` is true when it should be false.

NOTE

- this PR depends on #1941 (resolved)

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Commit messages:
 - Merge branch 'master' into 8371183.is.edit
 - Merge branch 'master' into 8371183.is.edit
 - javadoc
 - Merge remote-tracking branch 'origin/8366201.allow.undo' into 8371183.is.edit
 - javadoc
 - is edit
 - Merge remote-tracking branch 'origin/8366201.allow.undo' into 8371183.is.edit
 - suggestion
 - set text
 - Merge branch 'master' into 8366201.allow.undo
 - ... and 16 more: https://git.openjdk.org/jfx/compare/f8bef157...49c78747

Changes: https://git.openjdk.org/jfx/pull/1961/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1961&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371183
  Stats: 63 lines in 5 files changed: 46 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jfx/pull/1961.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1961/head:pull/1961

PR: https://git.openjdk.org/jfx/pull/1961